238 MANUFACTURE OF 



Two toys are made of unannealed glass, which, 

 though commonly used for the amusement of chil- 

 dren, exhibit phenomena which justly interest the 

 curiosity of the philosopher; we mean Prince 

 Rupert's drops, and the* Bologna flask or philoso- 

 phical phial. 



Prince Rupert's Drops are made by letting drops 

 of melted glass fall into cold water : the drop as- 

 sumes, by that means, an oval form, with a tail or 

 neck resembling a retort. These drops are said to 

 have been first invented by Prince Rupert, and 

 are, therefore, called by his name. They possess 

 this singular property, that if a small portion of the 

 tail is broken off, the whole bursts into powder with 

 an explosion ; and a considerable shock is commu- 

 nicated to the hand that grasps it. 



The Bologna or philosophical phial, is a small 

 vessel of glass, which has been suddenly cooled, 

 open at the upper end, and rounded at the bottom. 

 It is made so thick at the bottom, that it will bear 

 a smart blow against a hard body, without break- 

 ing ; but if a little pebble, or piece of flint, is let 

 fall into it, it immediately cracks, and the bottom 

 falls into pieces : but unless the pebble or flint is 

 large and angular enough to scratch the surface of 

 the glass, it will not break. 



The most generally received explanation of these 

 facts is fdunded on the assumption, that the dimen- 

 sions of those bodies which are suddenly cooled, 

 are larger than those which are more gradually 

 cooled. The dimensions, therefore, of the smooth 

 external surface of these glasses which are suddenly 

 cooled, are supposed to be larger than is adapted to 

 the accurate envelqpement of the internal part, 

 which is necessarily cooled in a more gradual man- 



